1. Field of the Invention
The present invention relates generally to anaerobic digestion of organic acids and alcohols into hydrogen and carbon dioxide and further conversion thereof to methane. Specifically, the present invention relates to a process for converting organic acids and alcohols into precursor metabolites for biological methane synthesis using photosynthetic bacteria and radiant energy.
2. Description of the Prior Art
Methane production by anaerobic fermentation (also called anaerobic digestion) involves the conversion of organic matter at modest temperatures, ambient pressures, and nearly neutral pH to methane and carbon dioxide in the absence of exogenous electron acceptors such as oxygen, nitrate, and sulfate through a series of microbial interactions. Conventional anaerobic digestion is used for waste water treatment. The rate of production of methane in such digestion, however, is usually too low to be an economically competitive process for production of methane from purchased substrates. Anaerobic digestion has been used for decades for waste treatment and disposal and as a source of fuel gas, particularly in developing countries.
Methanogens (i.e., methane-producing bacteria) have been studied for their utility in digestion processes for production of methane. To degrade complex organic substrates to methane by anaerobic digestion other organisms are necessary because of the limited number of substrates catabolized by methanogens. Such organisms are the fermentative species that convert carbohydrates, proteins, and lipids to lower molecular weight materials. These materials are then utilized by hydrogen-producing acetogenic bacteria to form acetate and hydrogen for consumption by the methanogens. Another group of acetogenic bacteria converts hydrogen and carbon dioxide to acetate and other acids.
Because there is a wide variety of organic structures in complex substrates, many different bacterial species are required to facilitate degradation of such structures. The fermentative bacteria found in operating methane fermentations supplied with complex substrates have been reported to be obligate anaerobes such as Bacteroides, Clostridium, Butyrivibrio, Eubacterium, Bifidobacterium, and Lactobacillus.
The first step in fermentation of complex substrates is extracellular enzyme-catalyzed hydrolysis of polysaccharides to oligosaccharides and monosaccharides, proteins to peptides and amino acids, triglycerides to fatty acids and glycerol, and nucleic acids to nitrogen heterocycles, ribose and inorganic phosphate. The sugars are degraded through pyruvate to acetate, higher fatty acids, carbon dioxide, and hydrogen. The amino acids and glycerol are degraded by the glycolysis pathway to the same products. After hydrolysis and glycolysis, some of the fermentation products are suitable substrates for the methanogens, but most are not.
Conventional degradation of the substrates not suitable for methanogens is caused by another group of anaerobes called acetogenic bacteria. These bacteria convert the alcohols and higher acids to acetate, hydrogen, and carbon dioxide An example of this type of bacteria is the S organism from the "Methanobacterium omelianskii" consortium.
It has been previously recognized that at least three groups of bacteria are involved in anaerobic digestion. Fermentative bacteria accomplish hydrolysis and conversion of the substrates to intermediates and their transformation to acetate, higher acids, hydrogen, carbon dioxide, and other low molecular weight compounds. Additional acetate, hydrogen, and carbon dioxide are produced by the acetogenic bacteria, and the methanogenic bacteria yield methane and carbon dioxide from acetate, and methane and water from hydrogen and carbon dioxide. Methane fermentation can be carried out in batch, semicontinuous or continuous processes.
In efforts to increase the economical production of methane using anaerobic digestion processes, various techniques have been attempted. These efforts have included adding high loadings of solids; immobilizing bacteria on solid supports; adding materials such as activated carbon, fly ash, enzymes, cultures and growth factors; pretreating the solids chemically or physically; and designing various types of digesters.
The slow steps of the soluble reactions that occur in anaerobic digestion are the conversion of organic acids into hydrogen, carbon dioxide, and acetate, and also the conversion of acetate into methane. This apparently is due to energy constraints.
Analysis of batch anaerobic digesters indicates that only 4% of the methane produced results directly from the hydrogen and carbon dioxide generated during fermentation. Twenty percent results directly from fermentative acetic acid and seventy-six percent results indirectly from other organic acids following obligate proton-reducing acetogenesis. The separate steps are shown in FIG. 1.
Methane production from organic acids, including acetic acid, is slow compared to that resulting from hydrogen and carbon dioxide. The slow conversion of organic acids into methane precursors is understandable from energetic bases, because little free chemical energy becomes available. The reactions proceed in the direction of organic acid breakdown only because the hydrogen product is scavenged to very low concentrations by methanogenic bacteria. With this necessary requirement for low steady-state levels of hydrogen, methane production is far less than maximal.
It has been reported that average digester rates of about one volume of methane produced per equal volume of liquid per day (VVD) can be increased to about 20-40 VVD when supplemental hydrogen is provided, and it can be increased to 220 VVD when hydrogen at high pressures to effect increased mass transfer of gas is combined with cell recycle. If hydrogen is placed in solution the requirement for high pressures is not necessary.
Previously, there has been no technique or process for simple and effective increase in methane production in anaerobic digestion processes.